The present invention relates to an ATM switching system used for broadband ISDN, or more in particular to an ATM switching system suitably used for burst data communication service carried out in Permanent Virtual Connection (PVC) mode.
The communication service of the broadband ISDN is either of two modes; Switch Virtual Connection (SVC) and PVC.
In SVC mode, (Virtual Path Identifier (VCI)/Virtual Channel Identifier) is assigned to a call in the ATM switching system at the time of setting (connecting) the particular call to establish a routing path while at the same time securing a bandwidth of the particular call. As a result, in SVC mode, the amount of incoming cells for each output line of the ATM switch is accommodated in a predetermined bandwidth, and therefore the storage capacity of the output cell buffer provided for each output line can be comparatively small.
In PVC mode, by contrast, VPI/VCI is assigned fixedly between specific communication terminals so that the terminal equipment can communicate on a dedicated line whenever required. When the communication is in PVC mode, the bandwidth is not secured for the VPI/VCI unlike in the ATM switching system in SVC mode. In the case where a plurality of burst data in PVC mode directed to the same output line are applied at the same time to the ATM switch, therefore, excessive ATM cells beyond the bandwidth of the output line flow into the buffer memory associated with the output line, and the excess cells beyond the buffer capacity are unavoidably discarded.
The cell discarding can be avoided by two methods. In a method, a sufficient buffer capacity is provided to accommodate a plurality of burst data generated for the same output port. The other method is such that the required bandwidth is secured in each system on the data route each time before transmission of the burst data from the PVC mode terminal.
The burst data produced from the RAM or the hard disc of the terminal equipment may amount to as large as 1 Mbits to 1 Gbits. The method in which a buffer capacity sufficient to accommodate a plurality of burst data is secured in the ATM switch involves a vast amount of buffer memory capacity, and therefore is not of practical value from the viewpoint of memory utilization.
In the method in which a bandwidth is secured each time of burst data transmission in PVC mode, by contrast, the time required for securing the bandwidth is not negligible, and the problem is posed of a reduced communication efficiency of the terminal equipment.
Assuming that an ATM switching system is employed in which control information including call control and bandwidth control are gathered in a processor and various control operations are performed in compliance with commands from the processor. In the case where a broadband ISDN is configured which is capable of transferring data of about 10 Mbits at the bit rate of 150 Mb/s within 100 ms, for example, the time required for securing a bandwidth exceeds the time required for data transfer, so that the overhead time for securing a bandwidth may pose a bottleneck to the burst data communication. This overhead time can be reduced to some degree by improving the processor capacity. This improvement based on the processor capacity, however, has its own limit.
There have been proposed various conventional techniques, such as U.S. Pat. No. 5,124,977 and U.S. Pat. No. 5,184,346 both related to a switching system using a common buffer memory U.S. Pat. No. 5,099,475 related to a switching system in which a high speed line is connected to the input and output links of an ATM switch through a cell multiplexer or cell demultiplexer, and U.S. Pat. No. 5,280,475 related to traffic shaping method and circuit in which a list structure is formed at every virtual path in a common buffer memory to control the reading of cells by a band control table.
An object of the invention is to provide an ATM switching system and an ATM cell control method which are improved in a way suitable for burst data communication.
Another object of the invention is to provide an ATM switching system and an ATM cell control method in which a buffer memory of comparatively small capacity can be used for each output line, and the burst data in PVC mode can be communicated efficiently.
In order to achieve the above-mentioned objects, according to the invention, there are provided an ATM switching system and an ATM cell control method, wherein in the case where a plurality of burst data directed to the same output line flow into the switching system in a temporally overlapped fashion. Only those cells associated with one or a plurality of burst data specified with the arrival of the leading cell thereof are admitted, while the cells belonging to the other burst data are discarded.
Cell admission or discarding is determined in the following manner. All the cells associated with burst data with the leading cell thereof arriving when the bandwidth of the output line has room are admitted, whereas the cells of burst data with the leading cell thereof arriving when there is no room in the bandwidth are all discarded.
More specifically, in the system according to the invention, state information is stored indicating the state of use of the output bandwidth of each output line or the use thereof by other burst data. At the time point of arrival of the leading cell of each burst data, part of the header information of the leading cell is registered as identification information for admitted burst data and the leading cell of the admitted burst data is stored temporarily in a buffer memory, if the transmission of the particular burst data can be permitted according to the state information. In the case where the bandwidth of the output line is in a state that cannot permit transmission of the burst data at the time point of arrival of the leading cell, on the other hand, the particular leading cell is discarded.
When a cell other than the leading cell of a burst data has arrived, a decision is made as to whether it should be admitted or discarded according to whether the header of the cell contains the burst identification information registered as described above. The registration ration of the identification information of an admitted burst data is cancelled when the last cell of the particular burst data is processed.
The identification information used for an admitted burst data include the identification information contained in the header of the leading cell of the burst data, such as VCI (Virtual Channel Identifier), VPI (Virtual Path Identifier), a combined value of VPI and VCI, part of VPI part of VCI or part of the combined value of VPI and VCI.
According to one aspect of the invention, in the case where a plurality of burst data directed to the same output port have arrived in temporally overlapped fashion, whether the burst data is admitted or not is determined at the time of arrival of the leading cell of the burst data. When a burst data is rejected, the subsequently-arriving cells of the particular burst data are all discarded, while all the cells of an admitted burst data arriving subsequently are admitted. As a result, the cell discarding operation caused by congestion is prevented from affecting all the burst data, thereby reducing the number of burst data which are required to be retransmitted due to the cell discarding.
According to another aspect of the invention, all the subsequently-arriving cells of a burst data which is rejected are discarded, and therefore the capacity of the buffer memory required for temporarily storing the output cells for each output line can be reduced.
The foregoing and other objects, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in conjunction with the accompanying drawings.